Objects (substrates) to be coated that require high levels of appearance, such as automobiles, are generally painted using methods that employ atomization of coating compositions, in consideration of, for example, the appearance of the coating film formed, and production efficiency. Spray painting and rotational atomizing painting are specific examples of such painting methods.
Generally, the coating composition used for atomization painting should preferably have low viscosity, and form small granules when released in a spray or when atomized as in, for example, rotational atomization, in order to produce a coating film with excellent smoothness. Once applied to the substrate to be coated, the coating composition should preferably have high viscosity, which discourages mixing with the overlaid coating composition and produces a coating film with excellent distinctness of image (hereinafter referred to as DOI). When the coating composition contains a luster pigment such as an aluminium pigment, the high coating composition viscosity after the composition has been applied to the substrate restricts the movement of the luster pigment in the coating composition, and thus produces a coating film that has good luster. Generally, a coating film with good luster is one in which brightness changes over a wide range depending on the viewing angle of the coating film, and in which the luster pigment in the coating film is relatively uniform, thus producing hardly any metallic mottling. Coating films that exhibit such large brightness changes depending on the viewing angle are generally described as having a high flip-flop property.
For these reasons, in terms of forming a coating film having superior appearance, it is preferable that the coating composition have low viscosity under high rate of shear as in atomizing, and high viscosity under low rate of shear after the application. In other words, it is preferable that the coating composition be one in which the viscosity decreases with an increase in rate of shear.
Such a coating composition that lowers its viscosity while increasing rate of shear can be produced using, for example, a method in which an associated thickener is blended into the coating composition. The associated thickener is generally a thickener that includes a hydrophilic portion and a hydrophobic portion per molecule, and that effectively exhibits a thickening effect with the network structure formed in an aqueous medium by the adsorption of the hydrophobic portion onto the surfaces of the pigments or emulsion granules in the coating composition, or by the association of the hydrophobic portions.
The associated thickener generally develops viscosity with the network structure formed by hydrophobic interaction. Associative strength is relatively weak in hydrophobic interaction, and as such the network structure is disrupted and the viscosity decreases upon applying a large shear force. Specifically, the coating composition containing the associated thickener has the characteristic of lowering its viscosity with an increase in rate of shear.
Recent concerns over environmental pollution caused by the vaporization of organic solvents have led to the development of aqueous coating compositions.
Aqueous coating compositions may typically include a surfactant used to disperse a hydrophobic resin component in water.
However, the use of the associated thickener in such aqueous coating compositions containing a surfactant is problematic, because the viscosity-developing effect of the associated thickener may become weak, and DOI or luster of the resulting coating film may be lowered. Specifically, because the aqueous coating composition has low viscosity upon being applied to the substrate to be coated, the coating composition may mix with the overlaid coating composition and lower DOI of the resulting coating film; or the luster pigments, when contained in the aqueous coating composition, may move in the coating composition upon application and disorient themselves, lowering the flip-flop property or causing metallic mottling. Attempts to increase the viscosity upon application by increasing the content of the associated thickener in the aqueous coating composition often result in a coating film with poor smoothness, because the increased thickener content also increases the viscosity under high rate of shear, and increases the size of the coating composition granules produced when atomizing the coating composition.
For example, Patent Literature 1 describes a viscosity-controlling agent that includes a hydrophobic portion and a hydrophilic portion, obtained by hydrophobic modification of a hydrophilic polymer, and/or hydrophilic modification of a hydrophobic polymer. The viscosity-controlling agent is described as being capable of reducing the concentration dependence of the aqueous dispersion viscosity. Further, according to this publication, an aqueous coating composition that uses the viscosity-controlling agent can exhibit stable flowability, and can stably produce a coating film with a good finish, regardless of the paint conditions, particularly the temperature and humidity conditions. However, viscosity development by the viscosity-controlling agent is not always sufficient. Specifically, sufficient viscosity may not be obtained when the viscosity-controlling agent is used in a coating composition that contains a surfactant. In this case, the resulting coating film may suffer from poor DOI and poor flip-flop property, or may produce metallic mottling.
Patent Literature 2 describes an associated thickener produced by adding one molecule of alkenyl succinic anhydride or alkyl succinic anhydride to a polyethylene glycol compound for each of at least two amino groups attached to the ends of the polyethylene glycol compound molecule. The associated thickener is described as being desirable as a thickener for latexes and aqueous coating compositions for its ability to improve the ease of solution adjustment without accompanying large viscosity changes following changes in the amount of associated thickener used, and to make quality control of the products easier. However, viscosity development by the associated thickener is not always sufficient. Specifically, sufficient viscosity may not be obtained when the associated thickener is used in a coating composition that contains a surfactant. In this case, the resulting coating film may suffer from poor DOI and poor flip-flop property, or may produce metallic mottling.
Patent Literature 3 describes an aqueous coating composition that includes an emulsion resin (A) of a specific structure, a specific viscosity-controlling agent (B), and a hydrophobic solvent (C). The aqueous coating composition is described as being capable of forming a coating film that has reduced metallic mottling, and excellent flip-flop property and smoothness. However, a coating film with sufficient DOI and luster, or a coating film with sufficient water resistance cannot always be obtained even with the use of this aqueous coating composition.